The present invention relates to a cathode unit for fluorescent tubes according to the preamble to claim 1. The invention also relates to the manufacturing industry for fluorescent tubes and to a method for manufacturing fluorescent tubes according to the preamble to claim 10. Similarly, the present invention relates to a fluorescent tube according to the preamble to claim 11, which fluorescent tube is designed for a long life.
Today, fluorescent tubes are manufactured with a long life as regards the operating time. WO 81/01244 describes a cathode unit comprising a cathode screen, also called an electrode screen, constructed as a cylindrical casing, which casing is connected to the end facing the discharge by means of a plate of electrically insulating material provided with a central hole. The design works very satisfactorily. However, further developments of the same have resulted in improvements, particularly regarding the adaptation of the cathode unit to narrow fluorescent tubes. It has been found that the plate does not necessarily need to be made of mica or other material that does not conduct electricity.
Fluorescent tubes of the abovementioned type comprise electrodes, which alternately work as cathodes and anodes, the cathode function constituting the critical factor, both as regards length of life calculated in operating hours, and product reliability. The electrode is provided with a special emitter material, which has an ability to emit electrons at a moderate temperature and energy supply. The emitter material comprises alkali oxides. The life of the electrode is limited by evaporation and sputtering of emitter material from the electrode's so-called hot spot. The hot spot obtains its heat initially from electrical heating and kinetic energy in the incident positive ions. The emission of electrons takes place from this spot. This means that the greatest concentration of ionised emitter material, such as barium, strontium and calcium, is found in the immediate vicinity and a few millimetres out from the hot spot. The task of the cathode screen is to increase the concentration of positive ions and in particular the ionised emitter material in the immediate vicinity of the electrode's hot spot.
A problem with known technology is that installation of the cathode unit according to the known embodiment in a narrow fluorescent tube body demands great precision. Similarly, the manufacture of a cathode unit consisting of several parts requires a large amount of work, which is costly.
There are currently no cathode units suitable for narrow fluorescent tubes which prolong the operating time of the fluorescent tube, while at the same time simplifying the manufacturing process. In addition, known cathode units can not be handled in mechanical manufacturing processes.
An object of the present invention is to avoid the said disadvantages of the known technology.
An additional object of the invention is to achieve a cathode unit that remains in working order, as far as the operation of the fluorescent tube is concerned, during the transportation of the fluorescent tube.
The abovementioned problems have been solved by means of the cathode unit described in the introduction, as described in the characterising part of Claim 1.
In this way it is possible to install the cathode unit in a narrow fluorescent tube more rapidly and in a more automated way, which is cost-effective. At the same time, the risk of damage to the coating on the inside of the fluorescent tube body during the manufacture of the fluorescent tube is reduced.
Alternatively, the cathode screen is designed with at least one side wall essentially incident to a centre line. By this means, the so-called pumping process for eliminating impurities in a fluorescent tube during manufacture can be made more efficient. Similarly, the installation of the cathode unit in the fluorescent tube body is made easier, while the tolerance is greater within the area of the incident side wall.
The cathode screen is preferably manufactured in one piece. The manufacture of the cathode screen can thereby be achieved in one stage which is cost-effective. Similarly, the cathode screen is made from only one component which eliminates the risk of malfunction caused by incorrect installation of components forming the cathode screen. The smaller the components, the more difficult it is to assemble these. The cathode screen manufactured in one piece prolongs the life of the fluorescent tube by eliminating the abovementioned malfunctions.
The cathode screen is suitably manufactured of metal which has a small tendency to react with the components of the atmosphere within the fluorescent tube. Such a metal is iron. In this way, the manufacture of a cathode screen can be made more cost-effective, as the metal is simple to shape and retains its shape after processing. The use of the pure metal, such as preferably pure iron, means that there are no chemical impurities which, if present, could cause reduced function of the cathode's emitter material. It has been shown by experiment that a cathode screen that is manufactured completely of pure metal, in which the central opening is approximately 5 mm in diameter, has the ability to collect and retain a large number of positive charged particles for a considerable time in the vicinity of the hot spot, which contributes to the return of the emitter material to the electrode.
Alternatively, the cathode screen is designed with at least one slot within the area of the said power supply device. The cathode screen can thereby be electrically insulated from the electrode even if, during transportation, the cathode screen comes to rest in a position that is displaced in relation to the centre line of the fluorescent tube. Similarly, the distance can be increased between the two power supply devices while retaining the insulation reliability. In addition, longer cathode spirals with more emitter material can be used, which prolongs the operating time of the fluorescent tube.
The cathode screen is preferably provided on the outside with a heat-insulating material. In this way, it is ensured towards the end of the life of the electrode that the cathode screen does not conduct heat to the wall of the fluorescent tube when the cathode screen is heated up by the electrode resulting in it being bent downwards by the force of gravity towards the wall of the fluorescent tube as a result of heating and softening of the device holding the cathode screen. The danger is thereby avoided of the fluorescent tube shattering and falling out of its mounting.
The outer side of the cathode screen viewed in the longitudinal direction of the cathode screen, suitably follows a straight line essentially parallel to the longitudinal axis of the said fluorescent tube body. A maximal amount of emitter material can thereby be applied to an electrode, whereby the life of the fluorescent tube is prolonged. This is to say, a cathode screen arranged centrally to the centre line of the fluorescent tube body and where the wall thickness of the cathode screen is even, means that both the input points of an electrode can be located at a maximal distance from each other inside the wall of the cathode screen. The cathode screen is placed at such a distance from the wall of the fluorescent tube body that there is no contact between them. The distance between the electrode and the inner side of the cathode screen is to be as small as possible in order for the desired effect to be obtained. However there must be no electrical contact between them.
Any occurrence of polluted gases in the discharge also has a de-ionising effect. The use of a cathode screen makes high demands on the design of the cathode unit, as the ignition of the fluorescent tube can be carried out more easily without the use of any cathode screen. This makes high demands on an elimination of the gaseous impurities in the fluorescent tube.
Alternatively, the second end of the cathode screen is completely open. During the manufacture of the fluorescent tube, various types of pump processes are used to remove the decomposition products of the emitter material. Effective pumping is particularly important for cathode units with the maximal amount of emitter material. The completely open second end ensures that satisfactory ventilation is achieved by the pump process for the removal of the decomposition products and other impurities. The life of the fluorescent tube is thereby prolonged. The completely open second end is also achieved in order to reduce the weight of the cathode screen, which reduces the risk of the cathode screen being displaced in a radial direction during transportation. The lower the weight, the less turning moment with the device holding the cathode screen acting as a lever, and the cathode screen can be held in position during the transportation. Similarly, the completely open second end allows the electrode to be inserted into the cathode screen in a simple way during the manufacture of the cathode unit.
The inner side of the cathode screen is preferably coated with an electrically-insulating material. The cathode screen can thereby be electrically insulated from the electrode even if, during transportation, the cathode screen comes to rest in a position that is displaced in relation to the centre line of the fluorescent tube body.
The abovementioned problems have been solved by means of the method described in the introduction, by the steps described in the characterising part of the claim 10.
In this way, the manufacture of the fluorescent tube is made more efficient. As the cathode screen is manufactured in one piece, time can be saved during the production, which is cost-effective. For the large amount of emitter material achieved according to the present invention, in relation to the relatively small space inside the cathode screen, the completely open opening at the second end of the cathode screen means that an efficient removal of the decomposition products can be carried out by the pumping process.
The abovementioned problems have similarly been solved by means of the fluorescent tube described in the introduction, as described in the characterising part of the claim 11. In this way, a narrow fluorescent tube has been achieved, for example the so-called T5, T4 and T3 fluorescent tube, which is simple to manufacture and which has a longer life in relation to known technology. The same technology can also be used for the T8 fluorescent tube.